The fact is so well-researched and understood by the general public, it has become accepted wisdom: Kids raised by their married, biological parents tend to be healthier (both mentally and physically) and do better in school. But why? Children of divorce and adopted children are loved no less deeply. To attempt to answer this question is, it turns out, to fall down a bit of a rabbit hole.

Is there something intrinsic to marriage itself that directly leads to better outcomes for children? At a time when so many American children are growing up with single or cohabiting parents, this is an important question for both policy-makers and parents. Marriage may not ultimately be for adults. Marriage may be, strange as it sounds, for children.

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When we look at the relationship between marriage and children’s wellbeing, the question of causality looms large. If children of married parents receive better parenting, for example, does that mean that marriage leads to good or successful parenting or does it mean that people with the traits of good parents are more likely to marry and stay married?

Researchers call this a selection problem. In other words, it’s possible that married parents tend to select marriage because they have certain qualities — higher incomes, more education, larger social networks — that also tend to produce better outcomes for children. Sophisticated statistical techniques can help sort out this problem and researchers have done just that, examining the mechanisms through which marriage might improve children’s lives, looking for causal effects.

For a recent issue of the Future of Children, I reviewed research on a variety of mechanisms that might explain why children of married parents fare better than other children. Some of the mechanisms have been well studied, including parents’ income, fathers’ involvement with their children, parents’ physical and mental health, parenting quality, health insurance, home ownership, parents’ relationships, and family stability. Others have received less attention, including net wealth, constraints on borrowing, and informal insurance through social networks.

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All of these mechanisms tend to vary by family structure (that is, whether the children have married parents or live in another family arrangement). All of them may affect some aspect of children’s wellbeing, such as health or educational attainment. Yet when researchers study them, they typically find that a given mechanism explains some but not all of the relationship between family structure and children’s outcomes.

For example, a recent study hypothesized that higher household income and greater access to health insurance might explain why children of married parents are healthier. This logical hypothesis was confirmed by the authors who found that family structure was associated with income and insurance, and that income and insurance were in turn associated with children’s health. But the finding was complicated by the fact that, even among children with similar household income and similar access to health insurance, those whose parents were married were also healthier. In other words, the researchers found support for their hypothesis that differences in income and insurance produced differences in children’s health, but they also found that family structure had associations with health beyond income and insurance. This pattern of partial explanation is repeated across many, many studies.

The principal exception to this pattern comes from studies of family stability. When researchers measure instability by the simple number of transitions between different family arrangements (for example, from living with married to parents to living with a single parent after a divorce to living in a stepfamily), they find that instability often accounts for most if not all of the associations between family structure and children’s outcomes. So stability could be the mechanism through which marriage improves children’s wellbeing. Still, it could also be that these studies haven’t really explained why family structure matters; rather, they may have just found that counting the number of transitions is the best way to measure family structure.

What can we conclude from the fact that almost wherever we look, mechanisms such as higher income, more education, better access to health insurance and so on don’t fully explain the association between American children’s wellbeing and marriage? One reasonable conclusion is that the advantages of marriage for children’s wellbeing will be hard to replicate through policies other than those that bolster marriage itself. While helping unmarried parents increase their incomes, spend more time with their kids, and find better child care would surely benefit children, these are likely to be, at best, only partial substitutes for marriage itself. The advantages of marriage for children appear to be the sum of many, many parts.

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Professor David C. Ribar is a Professor of Economics and Faculty Director of the Child and Family Policy Lab in the Andrew Young School of Policy Studies at Georgia State University in Atlanta, Georgia. He has conducted research on family formation, child care, the consequences of teenage fertility, the economic motivations behind public and private transfers, welfare reform, the administrative burdens of assistance programs, people’s time use, food insecurity, homelessness, and other topics.